Objective To analyze the types and epidemiological characteristics of influenza virus in Xi'an during 2009 to 2017.Methods A total of 21 856 samples of throat swabs from patients with influenza like illness ( ILI) were collected from 5 national influenza sentinel surveillance hospitals from August 2009 to December 2017.Influenza virus nucleic acid was detected by real-time fluorescence quantitative PCR and virus types were confirmed , chick-embryo cells or Madin-Darby canine kidney (MDCK) cells were used to isolate influenza virus.SPSS 18.0 software was used for data analysis.Results The positive detection rate of influenza virus was 16.19%(3 539/21 856), the seasonal influenza A virus subtypes including H1, H3, the new type H1and H7 accounted for 62.39%(2 208/3 539), influenza B virus subtypes including Victoria , Yamagata and unclassified type B accounted for 37.50%( 1 327/3 539), and the mixed influenza virus infection accounted for 0.11%(4/3 539).The positive rate of influenza virus detected in different years was significantly different ( χ2＝357.651, P ＜0.01).During January to March the major influenza A viruses accounted for 49.07%(947/1 930), influenza B viruses accounted for 50.93%(983/1 930); during October to December , the influenza A viruses accounted for 78.07%( 1 061/1 359 ), and influenza B viruses accounted for 21.93%( 298/1 359 ); there was significant difference in composition of type A virus and type B virus between different seasons ( χ2＝ 550.06, P＜0.05).The positive detection rate of influenza virus in patients with ILI of age groups 0-3 years,＞3-7 years,＞7-13 years,＞13-18 years,＞18-24 years,＞24-60 years and ＞60 years were 12.61%, 19.41%, 19.66%, 22.98%, 14.91%, 13.50% and 12.84%, respectively ( χ2＝202.52, P＜0.05).Conclusion Influenza A virus is common in Xi'an,winter and spring are the peak seasons for influenza epidemics.It is recommended for susceptible people to take influenza vaccination .